1. Field of the Invention
This invention relates to a fuel assembly, and more particularly to a fuel assembly suitable for the boiling water reactor.
2. Description of the Prior Art
Uranium 238 contained in the nuclear fuel material changes into plutonium 239 by absorption of the high-speed neutron. With an increase in void fraction in the core, a quantity of high-speed neutron increases. Plutonium 239 causes fission by absorbing thermal neutron. With a decrease in the void fraction in the core, a quantity of the thermal neutron increases, too. Spectral shift operation utilizes the above characteristics. The spectral shift operation has the following features:
(1) At the beginning of the fuel cycle, plutonium 239 is produced by increasing the void fraction in the core. PA0 (2) At the end of the fuel cycle, fission of plutonium 239 is increased by lowering the void fraction. The nuclear fuel material is used efficiently by the spectral shift operation.
EP-A-0282600 shows the fuel assembly with the spectral shift rod which is one example to realize the spectral shift operation.
This spectral shift rod is a water rod which comprises a coolant descending path that leads cooling water rising inside of a coolant ascending path to the lower part of the core. The coolant ascending path has a cooling water inlet port. This cooling water inlet port communicates with the lower part which is located at a portion lower than the fuel supporting portion of the lower tie-plate. The coolant descending path has a cooling water outlet port that communicates with the cooling water passage formed among the fuel rods in the upper part of the fuel supporting portion.
The above water rods have a function of changing a first condition of forming a steam area inside the coolant ascending path (a coolant surface is formed in the coolant ascending path) to a second condition of filling inside of the coolant ascending path with cooling water (a coolant surface is not formed in the ascending path), by setting the resistance value of the fuel supporting portion to the prescribed value, as shown in EP-A-0282600, according to the increase of the cooling water flow rate that is supplied in the fuel assembly.
The spectral shift operation can be achieved by the operation of forming the steam area inside water rods and not forming it inside the water rods. This spectral shift operation can provide a great improvement of a fuel economy. The resistance value at the fuel supporting portion is defined as a rate of the total cross-sectional area of the plural penetrating holes to the cross-sectional area of the cooling water passage formed among the fuel rods.